A channel layer of a thin film transistor can be made of metal oxide semiconductor. An etching stop layer can be arranged on the channel layer to protect the metal oxide semiconductor. A thickness of the etching stop layer is generally greater than 100 nanometers. However, in etching stop (ES) process a resolution of exposing a through hole in the etching stop layer is not high enough to achieve a shorter channel length between a source electrode and a drain electrode of the thin film transistor.